32 - Mercury (Hg) Species Transformation Dynamics in Surface Waters of Two Contrasting European Coastal Seas

Coastal regions receive significant continental Hg and nutrient input, potentially favoring monomethyl-Hg (MMHg) production and bioaccumulation in fish. An earlier study suggested the open sea as a major source of MMHg to a coastal area. However, to study Hg transformation dynamics in coastal environments, a methodological experimental approach is needed. Incubation experiments with enriched stable isotopic tracers have been successfully applied to determine Hg-transformation rates. We conducted species-specific enriched stable isotope incubation experiments (10 pM 199Hg(II), 1 pM 201MMHg) to determine methylation, oxidative/reductive demethylation and reduction rates following established protocols. The relative importance of (a-)biotic transformations was addressed by incubating (un-)filtered surface water samples from a coastal (C), shelf (S) and deeper margin station (O) in the western Mediterranean Sea (MS) and coastal (C) and estuarine-influenced (E) waters from the Bay of Biscay (BB, Atlantic Ocean) in fall 2020 and spring 2021. Methylation was not observed, however, our results indicate differences in demethylation dynamics between sites. (Photo)-demethylation in unfiltered waters was comparable for both sites (3.6-9.6x10-3h-1). In the dark, demethylation was significantly different in MS waters (fall; C: 3.5x10-4h-1, S: 4.6x10-5h-1) but comparable in BB waters (spring; E: 6.5x10-4h-1, C: 4.7x10-4h-1). Moreover, we observed reductive demethylation (spring; MS, BB) under dark conditions in unfiltered waters, absent or reduced in filtered waters, suggesting a biologically mediated pathway. Reduction of Hg(II) was found under all light conditions (max 4.3%h-1) but was considerably lower in the dark for off-shore MS waters (S, O; max 0.3%h-1). Faster MMHg degradation (photochemical, biological) in coastal waters suggests that concentration differences between coastal and off-shore waters can be explained by degradation dynamics.

Authors: Alina Kleindienst, Bastien Duval, Emmanuel Tessier, Lars-Eric Heimbuerger-Boavida, Corinna Schrum, Remy Guyoneaud, David Amouroux